'11^ 



*rhese values correspond very well with the pressures found b)* 

 ScHENCK and Rassbach witli a similar mixture and with a mixture 

 of PbS + PbO' which has been heated above 800° and then cooled. 



On openin" the apparatus it appeared that the reaction product, 

 although not fused, had strongly caked: the porcelain boat was 

 strongly attacked and on the rod a sublimate of very beautiful 

 PbS-crystals had again deposited. It was not doubtful that the 

 reaction mass contained metallic lead; there could be found large, 

 soft paper- marking and malleable particles. Finally, it was proved 

 by extracting a portion of the reaction product tirst a few times 

 with ammonium acelate and then with lead acetate. All the PbSO^ 

 and Pb() then dissolves. The residue was treated with fuming HNO3, 

 which converts the PbS quaiititaliveiy into PbSO^. After expelling 

 the HNO3 and filtering off the PbSO^, any Pb formed eventually as 

 Pb(N03)g mnst be present in the filtrate. 



The filtrate gave a strong lead reaction. The reaction mass thuj5 

 contains metallic lead. 



Hence Pb -\- PbO.PbSO^ are the stable phase pairand the ^pressures 

 measured relate to the reaction : 



2 PbO.PbSO, + 3 PbS = 7 Pb + 5 SO, . . . . (7) 



14. From the above it follows that with a sufficient excess of 

 basic lead sulphate the end of reaction (7) will be a mixture of 

 PbO . PbSO, and Pb. (In the latter, however, a little PbS will still 

 be dissolved). 



'I'his equilibrium is divariant and, on a sufficient reduction of 

 pressure, will pass into a monovariant equilibrium. 



The third condensed phase occurring therein cannot be a secopd 

 metallic phase for the solution of PbS in Pb already present is 

 mixable with pure lead in every proportion. 



Hence, it must be the basic sulphate (PbO), . PbSO, which follows 

 the PbO . PbSO, and the reaction occurring is indicated by the 

 following equation : 



Pb 4- 4 PbO.PbSO, = 3 (PbO), PbSO, + SO, ... (8 



The monovariant equilibrium of this reaction will in turn be 

 followed by still two other monovariant equilibria w^herein occur 

 the reactions represented by the equations : 



Pb + 5 (PbO), PbSO, = 4 (PbO), PbSO, + SO, . . (9) 

 Pb + (PbO),PbSO, = 5 PbO + SO, .... (10) 

 In these reactions primary formed lead therefore disappears on 

 behalf of PbO until, finally, only Pb + PbO is. left. 



